It is sought more particularly here below in this document to describe problems existing in the field of seismic data acquisition for oil prospecting industry. The disclosure of course is not limited to this particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
The operations of acquiring seismic data on site conventionally use networks of sensors (here below designated as “hydrophones” with regard to the acquisition of data in a marine environment). The hydrophones are distributed along cables in order to form linear acoustic antennas normally referred to as “streamers” or “seismic streamers”. As shown in FIG. 1, the network of seismic streamers 20a to 20e is towed by a seismic vessel 21. The hydrophones are referenced 16 in FIG. 2, which illustrates in detail the block referenced C in FIG. 1 (i.e. a portion of the streamer referenced 20a).
The seismic method is based on analysis of reflected seismic waves. Thus, to collect geophysical data in a marine environment, one or more submerged seismic sources are activated in order to propagate omni-directional seismic wave trains. The pressure wave generated by the seismic source passes through the column of water and insonifies the different layers of the sea bed. Part of the seismic waves (i.e. acoustic signals) reflected are then detected by the hydrophones distributed over the length of the seismic streamers. These acoustic signals are processed and retransmitted by telemetry from the seismic streamers to the operator station situated on the seismic vessel, where the processing of the raw data is carried out.
A well-known problem in this context is the localization of the seismic streamers. Indeed, it is important to precisely locate the streamers, in particular for:                monitoring the position of the hydrophones (distributed along the seismic streamers) in order to obtain a satisfactory precision of the image of the sea bed in the exploration zone;        detecting the movements of the streamers with respect to one another (the streamers are often subjected to various external natural constrains of variable magnitude, such as the wind, waves, currents); and        monitoring the navigation of streamers, in particular in a situation of bypassing an obstacle (such as an oil barge).        
In practice, it is aimed to carry out an analyze of sea bed with a minimum number of passage of the vessel in the concerned area. For that purpose, the number of streamers implemented in the acoustic network is substantially raised. The aforesaid problem of localization of the streamers is thus particularly noticeably, especially in view of the length of the streamers, which may vary between 6 and 15 kilometers, for example.
Control of the positions of streamers lies in the implementation of navigation control devices, commonly referred to as “birds” (white squares referenced 10 in FIG. 1). They are installed at regular intervals (every 300 meters for example) along the seismic streamers. The function of those birds is to guide the streamers between themselves. In other words, the birds are used to control the depth as well as the lateral position of the streamers. For this purpose, and as illustrated in FIG. 2, each bird 10 comprises a body 11 equipped with motorized pivoting wings 12 (or more generally means of mechanical moving) making it possible to modify the position of the streamers laterally between them (this is referred to a horizontal driving) and drive the streamers in immersion (this is referred to a vertical driving).
To carry out the localization of the seismic streamers (allowing a precise horizontal driving of the streamers by the birds), acoustic nodes are distributed along the streamers. These acoustic nodes are represented by hatched squares, referenced 14, in FIGS. 1 and 2. As shown in FIG. 1, some acoustic nodes 14 of the network are associated with a bird 10 (case of FIG. 2), and other are not.
The acoustic nodes 14 use underwater acoustic communication means, hereafter referred to as electro-acoustic transducers, allowing to estimate the distances between acoustic nodes (named here below “inter-node distances”). More specifically, these transducers are transmitters and receivers of acoustic signals, which can be used to estimate an inter-node distance separating two acoustic nodes (acting as sender node and receiver node respectively) situated on two different streamers (which may be adjacent or not) as a function of an acoustic signal propagation duration measured between these two nodes (i.e. a travel time of the acoustic signal from the sender node to the receiver node). From the acoustic network, this thereby forms a mesh of inter-node distances allowing to know precise horizontal positioning of all the streamers.
Transducer here is understood to mean either a single electro-acoustic device consisting of a transceiver (emitter/receiver) of acoustic signals, or a combination of a sender device (e.g. a pinger) and a receiver device (e.g a pressure particle sensor (hydrophone) or a motion particle sensor (accelerometer, geophone . . . )).
Usually, each acoustic node comprises an electro-acoustic transducer enabling it to behave alternately as a sender node and a receiver node (for the transmission and the reception, respectively, of acoustic signals). In an alternative embodiment, a first set of nodes act only as sender nodes and a second set of nodes act only as receiver nodes. A third set of nodes (each acting alternately as a sender node and a receiver node) can also be used in combination with the first and second sets of nodes.
The inter-node distance dAB between two nodes A and B can be typically estimated on the basis of the following formula: dAB=c·tAB, with:                node A acting as a sender node which transmits an acoustic signal S to node B acting as a receiver node (see example in FIG. 1, with acoustic signal S shown as an arrow between nodes referenced A and B);        tAB, the propagation duration (travel time) elapsed between the emission instant and reception instant of the acoustic signal transmitted from the sender node A to the receiver node B (assuming that the receiver node and the sender node are synchronized); and        c, a “measured” or “estimated” value of sound speed (also referred to as sound velocity) of the acoustic signal.        
Computation of an inter-node distance can be carried out, either by the navigation system (for positioning the set of hydrophones), or the node manager system (for providing useful information to the birds for horizontal driving), or the acoustic nodes themselves (in case they are equipped with electronics intended for this computation). The acoustic nodes are further synchronized by the node manager system through a wire communication bus placed within the streamers.
In the known methods for estimating an inter-node distance, the sound speed c which is used is supposed to be constant in the vertical plane. However, in practice this will not be the case. The sound speed in the ocean widely depends on the temperature, pressure and salinity of water (especially) and thus is almost always depending on depth (z) considered; in that case we talk about sound speed profile (SSP) c(z).
The shape of the sound speed profile in the area where the seismic survey is performed can modify the acoustic paths of sound. The sound will not follow a straight line (as supposed in the inter-node distance estimation method described above) but a curved ray path due to the refraction phenomena (according to Snell Descartes laws). Indeed, in a non uniform medium the sound rays can be bended (refracted) due to the change of the sound speed and more precisely to its gradient. The wavefronts of the sound are refracted toward the layer where the sound speed is lower, the refraction will be more pronounced if the change in the sound speed is rapid.
FIGS. 3 to 5 illustrate the influence of a sound speed gradient in the channel. For each of these figures, the left part presents a sound speed profile and the right part presents the corresponding ray paths, obtained with a ray path tracing method for a 10° aperture launch, and a 300 m-distance. Those figures enable to compare the ray paths followed by the sound in two mediums.
As can be seen in the left part of these figures, the first medium (FIG. 5) is a 50 m depth water column with a constant sound speed and the second medium (FIGS. 3 and 4) is a medium constituted with a 50 m water column and a 25 m depth minimum of sound speed with a constant gradient.
As can be seen in the right part of these figures, the depth of the source (sender node) is 25 m in FIGS. 3 and 5, 30 m in FIG. 4. The sound will follow straight paths in the first case (FIG. 5), and strongly curved paths in the second case, depending on depth (FIGS. 3 and 4).
When the path is curved, the distance along the path will be more important than in the straight line case. Thus the inter-node distance obtained with the previous method (assuming a constant sound speed profile) will be over estimated which is a synonym of a lack of localization precision or a bias in the localization result (the localization of the streamers being based on the inter-node distances obtained with a plurality of couples of acoustic nodes).
As described in the previous paragraphs, the sound speed value which is used, in the known methods, to estimate the inter-node distance is supposed to be constant in the vertical plane, which is usually a wrong assumption. Moreover, the environmental conditions (temperature, pressure or salinity of water), can change in a fast way depending on position and on weather conditions (sea state, sun influence, currents etc. . . . ). The shape of the sound speed profile can thus imply refraction phenomena which curves the ray paths. The classical formula used to estimate the inter-node distance (dAB=c·tAB) will not be valid any more and the travel time tAB will be a travel time on a curve (i.e. an arc length LAB) and not on a straight line.
Assuming a constant sound speed, an error on this sound speed value will imply a small error on the estimated distance between two close nodes. For instance, for an inter-node distance dAB=300 m, a 0.5 ms-1 error (classical value for a sound velocimeter) is equivalent to a 10 cm error on the inter-node distance. On the opposite, assuming for example the sound speed profile of the left part of FIG. 6 (50 m water column and a 25 m depth minimum of sound speed with a constant gradient), and a 15 m depth source, the direct path is illustrated in the right part of FIG. 6 (obtained with a ray path tracing method for a 10° aperture launch, and a 300 m-distance). The direct path length is equal to 300.70 m, which correspond to a 70 cm error on the inter-node distance when assuming a constant sound speed of 1482 ms-1 (at 15 m-depth) and a real distance of 300 m.
Moreover, if the two nodes A and B considered are not at the same depth, the ray path from node A to node B and the one from node B to node A can be different and so the travel time can be different depending on the way of the signal.
As shown in FIG. 7, in warm ocean region, a typical sound speed profile has three parts corresponding to the three layers of the water column: the surface layer (mixed layer), the main thermocline and the deep isothermal layer. The mixed layer can be few meters thick, but can also extend to several dozens of meter (depending on seasons, sun, sea state, currents . . . ). The mixed layer can disappear in colder oceans. The sound speed is almost constant for the mixed layer, but not for the main thermocline and the deep isothermal layer. The tendency in the field of seismic data acquisition is to increase the depth of the streamer which can place the streamer (and the acoustic nodes) under the mixed layer (and therefore in the main thermocline) and thus increase the refraction phenomena. As detailed above, this refraction phenomena causes an error if the classical formula is used to estimate the inter-node distance.